1. Field of the Invention
Proper control of the air-fuel mixture supplied from carbureter to the engine is very important in engine operation. This is always required in the case when it is desired to obtain the utmost effect of an improved engine devised to cope with automotive exhaust gas problems or when it is desired to obtain optimum catalytic purification of the exhaust gas of an engine provided with a catalytic converter for purifying the exhaust gas. This invention relates to an air-to-fuel ratio control means, which can sufficiently meet the above requirements.
2. Description of THE Prior Art
Heretofore, in the above type of air-to-fuel ratio control means for carbureters it has been usual to adjust the carbureter such that it provides slightly richer air-fuel mixture than the theoretical one, while operating a control valve controlling the flow of air through a conpensating air passage provided separately from the main carbureter passage according to the output of a discriminating circuit discriminating whether the air-to-fuel ratio is on the richer side or leaner side of the theoretical ratio from the output of an air-to-fuel ratio detector provided in the exhaust system to the value required for the catalytic exhaust gas purifier or the like provided in the exhaust system.
In such system, the air-to-fuel detector begins to provided its proper function when a certain termperature, i.e., activation temperature is reached. This temperature differs with different detectors. For example, the activation temperature of an air-to-fuel detector using metal oxide based in zirconium dioxide is as high as about 400.degree. C. With this detector the intended control of the air-to-fuel ratio cannot be obtained at the time of starting of the engine, particulalry at the time of cold starting (starting of the engine before it is warmed-up). Also, where the control valve is a butterfly valve rotatable within a limited angle as shown in FIG. 2, inconvenience is encountered in case when the required air-to-fuel ratio of the air-fuel mixture is not provided in the cold engine state or even when the fully closed position of the valve is reached. In such case, the discriminating circuit continually supplies a signal for driving the butterfly valve to the drive motor. Consequently, the drive motor tries to further rotate the butterfly valve, so that torsional rupture of the butterfly valve or trouble of the drive motor itself is liable to result. If the control valve is a butterfly valve without any limitation on the rotational angle as shown in FIG. 3, it would be to the "overrun" state, in which state the normal air-to-fuel ration control is no longer possible. If other valves than the above butterfly valves are used as the control valve, similar problems are encountered due to the use of stopper means from other factors.
Further. even in the normal operation of the above system, increasing the valve opening speed of the control valve leads to th problem of increased control width particularly under low-load low-speed engine operation conditions due to the delay time inherent in the whole system, although the air-to-fuel ratio more quickly converges to the required ratio under high-laod high-speed engine operating conditions or in transient engine states.